Near-field Interference Research Articles

Numerical Modeling and Experimental Investigation of the Breadboard Model of a Near-Field Interference Microscope

A circuit design for technology of near-field interference microwave microscopy using evanescent fields of several active radiators (probes) is suggested and substantiated. The feasibility of providing the spatial overlap of the evanescent fields of two probes representing expanding coaxial conical horns is demonstrated. It is shown that the structure of the electromagnetic field in the region of overlap can effectively be controlled by changing the phase difference of oscillations arriving at inputs of the probes. This technology can be used for diagnostics of liquid media comprising dielectric inhomogeneities, flaw detection, and hygroscopy.

Accurate Acoustic Signal Parameter Estimation for Marine Geodesy Surveys

ABSTRACTAcoustic signal parameter estimation is important for diverse marine geodesy surveys and several other applications. However, the received signal from a far-field target characterized by planar wavefront propagation is frequently affected by strong nearby interfering signals. Their presence deteriorates the performance of direction-of-arrival (DOA) estimation for far-field target. In order to enhance the reception of signal from far-field target, the near-field/far-field (NFFF) beamformer is proposed. Such a beamformer optimizes beam pattern for far-field detection by maximizing beamformer output in the direction of the far-field target with the imposed condition to eliminate interfering signals generated in near-field locations. As the interference suppression only occurs at the position of near-field interference, a possible blind zone for far-field detection in conventional methods is not created. The NFFF beamformer is applicable for coherent signals and the scenario with multi interferences. For stationary situation where interferers locations are fixed, the NFFF beamformer computations do not require time updates with associated computational load. Furthermore the proposed method can be extended to several new situations such as acoustic monitoring performed from a stationary platform subjected to water currents, waves, winds and other variables, all of them generating nearby interferences and also to different array configurations including 2D and 3D arrays.

Direction of Arrival Estimation Under Near-Field Interference Using Matrix Filter

Direction of arrival (DOA) estimation is of great interest due to its wide applications in sonar, radar and many other areas. However, the near-field interference is always presented in the received data, which may result in degradation of DOA estimation. An approach which can suppress the near-field interference and preserve the far-field signal desired by using a spatial matrix filter is proposed in this paper and some typical DOA estimation algorithms are adjusted to match the filtered data. Simulation results show that the approach can improve capability of DOA estimation under near-field inference efficiently.

Enhanced THz transmission and imaging of a subwavelength slit via light-induced diffraction

We present measurements and analytical modeling which demonstrate enhanced THz transmission through a subwavelength aperture via light-induced diffraction. Our experiment involves photoexciting a conducting pattern onto a silicon interface so as to control and modulate the near-field interference of THz radiation. To illustrate the concept, we photoexcite a simple double-conducting stripe pattern on the incident side of a silicon wafer which has a slit etched into a gold film on the exit side. We show that under certain resonant conditions set by the stripe dimensions, a constructive near-field interference can bring about enhanced transmission through the slit. By raster scanning the excitation pattern under these resonant conditions, one can build an image of subwavelength features such as the slit aperture of our sample.

Measuring the transverse spin density of light.

We generate tightly focused optical vector beams whose electric fields spin around an axis transverse to the beams' propagation direction. We experimentally investigate these fields by exploiting the directional near-field interference of a dipolelike plasmonic field probe placed adjacent to a dielectric interface. This directionality depends on the transverse electric spin density of the excitation field. Near- to far-field conversion mediated by the dielectric interface enables us to detect the directionality of the emitted light in the far field and, therefore, to measure the transverse electric spin density with nanoscopic resolution. Finally, we determine the longitudinal electric component of Belinfante's elusive spin momentum density, a solenoidal field quantity often referred to as "virtual."

Giant local circular dichroism within an asymmetric plasmonic nanoparticle trimer.

We investigated the near-field response in silver nanoparticle aggregates to the excitation of circular polarized light. In a right-angle trimer system, the local field intensity excited by right-hand circularly polarized light is almost one thousand times larger than the left-hand case. By analyzing the polarization and phase of the local field in plasmonic hotspots, we found this local circular dichroism is originated from the near-field interference excited by orthogonal polarized incident lights. The local circular dichroism can be tuned by the rotation of the third particle, the interparticle distance, and the dielectric environment. This phenomenon could also widely exist in more complicated nanoaggregates. These findings would benefit for resolving light handedness, and enhancing circular dichroism and optical activity.

Direction‐of‐arrival estimation for far‐field acoustic signal in presence of near‐field interferences

The far-field acoustic signal received by an acoustic array is frequently affected by near-field interferences. This causes deterioration of the direction-of-arrival (DOA) estimate for the far-field signal. To enhance the DOA estimate, the novel near-field/far-field (NFFF) beamformer is proposed. Such a beamformer optimises the beam pattern for far-field detection by maximising the beamformer output in the direction of the far-field target signal with the imposed condition to eliminate interfering signals from near-field locations. As the interference suppression only occurs at specific positions of near-field interferences, a blind zone in the far-field direction present in conventional methods will not be introduced. The NFFF beamformer is also applicable for coherent signals and for multi-interferers. For a stationary situation where interferers’ locations are fixed, the NFFF beamformer computations do not require time updates and the computational load is similar to that of the conventional beamformer. The method can be extended to several situations such as acoustic monitoring performed from a stationary platform subjected to water currents, waves, winds and other variables, all of them generating nearby interferences, and also to different array configurations including two-dimensional (2D) and 3D arrays.

Hydrodynamic optimization of a triswach

A new methodology for hydrodynamic optimization of a TriSWACH is developed, which considers not only the positions of the side hulls but also the shape of the side hulls. In order to account for the strong near-field interference effects between closely-spaced multihulls, an integrated hydrodynamic computational tool that consists of a potential-flow based simple CFD tool and an Euler/RANS/Navier-Stokes based advanced CFD tool has been further developed and integrated into a practical multi-objective hydrodynamic optimization tool. The other components of this hydrodynamic optimization tool consist of a hull shape representation and modification module and an optimization module. This enhanced multi-objective hydrodynamic optimization tool has been applied to the hydrodynamic design optimization of the TriSWACH for reduced drag by optimizing the side hulls only. A new methodology is developed to optimize side hull forms so that the TriSWACH has a minimal drag for a wide speed range and for various side hull positions. Two sets of the side hulls are developed and used for the design of two optimal TriSWACH models. Model tests are carried out for two optimal TriSWACH models at Webb Institute for validations. Substantial drag reductions have been obtained for a wide range of speed.

Vision-Based Alignment Control for Grating Tiling in Petawatt-Class Laser System

Large-sized diffraction gratings are required for the compressor of chirped-pulse-amplification systems. However, producing single large grating is of great difficulty, which makes grating tiling an alterative solution. This paper presents a vision-based alignment control method for grating tiling in petawatt-class laser system. Tiling errors with six degrees of freedom are grouped into three ones, and a decoupled vision control method is proposed. The piston error is measured from the far-field focal-spot image. To determine the piston error, the energy ratio between the main and secondary spots in the image is computed, and then a mapping formulation is determined to compute the piston from the energy ratio. To guarantee precision and reliability, a closed-loop piston control system with clustering error filtering and output verification is designed. The angular error is composed of two parts, i.e., tip and tilt errors, which is measured from the near-field interference fringe image. To reduce the influence of airflow on the angular measurement, a novel image quality estimation method is designed using the average crest-to-trough difference. Then, Fourier fringe pattern analysis is applied to the left and right parts of the image to reconstruct the wavefront, based on which a closed-loop grating angular control system is designed. Finally, experiments are well conducted in a tritiled gratings to verify the performance and precision of the proposed grating tiling system.

Sorting linearly polarized photons with a single scatterer

Intuitively, light impinging on a spatially mirror-symmetric object will be scattered equally into mirror-symmetric directions. This intuition can fail at the nanoscale if the polarization of the incoming light is properly tailored, as long as mirror symmetry is broken in the axes perpendicular to both the incident wave vector and the remaining mirror-symmetric direction. The unidirectional excitation of plasmonic modes using circularly polarized light has been recently demonstrated. Here, we generalize this concept and show that linearly polarized photons impinging on a single spatially symmetric scatterer created in a silicon waveguide are guided into a certain direction of the waveguide depending exclusively on their polarization angle and the structure asymmetry. Our work broadens the scope of polarization-induced directionality beyond plasmonics, with applications in polarization (de)multiplexing, unidirectional coupling, directional switching, radiation polarization control, and polarization-encoded quantum information processing in photonic integrated circuits.

Steady waves in a stratified flow over a combined obstacle

A problem of steady internal waves in subcritical flows of a stratified liquid over a finite sequence of bottom elevations is considered. An asymptotic solution of the second order of accuracy, which takes into account nonlinear effects, is constructed by the method of perturbations in terms of the small parameter of the obstacle height. Near-field interference wave structures are studied.

Ultra-broadband unidirectional launching of surface plasmon polaritons by a double-slit structure beyond the diffraction limit.

Surface-plasmon-polariton (SPP) launchers, which can couple the free space light to the SPPs on the metal surface, are among the key elements for the plasmonic devices and nano-photonic systems. Downscaling the SPP launchers below the diffraction limit and directly delivering the SPPs to the desired subwavelength plasmonic waveguides are of importance for high-integration plasmonic circuits. By designing a submicron double-slit structure with different slit widths, an ultra-broadband (>330 nm) unidirectional SPP launcher is realized theoretically and experimentally based on the different phase delays of SPPs propagating along the metal surface and the near-field interfering effect. More importantly, the broadband and unidirectional properties of the SPP launcher are still maintained when the slit length is reduced to a subwavelength scale. This can make the launcher occupy only a very small area of <λ(2)/10 on the metal surface. Such a robust unidirectional SPP launcher beyond the diffraction limit can be directly coupled to a subwavelength plasmonic waveguide efficiently, leading to an ultra-tight SPP source, especially as a subwavelength localized guided SPP source.

Mapping plasmonic near-field profiles and interferences by surface-enhanced Raman scattering

Mapping near-field profiles and dynamics of surface plasmon polaritons is crucial for understanding their fundamental optical properties and designing miniaturized photonic devices. This requires a spatial resolution on the sub-wavelength scale because the effective polariton wavelength is shorter than free-space excitation wavelengths. Here by combining total internal reflection excitation with surface-enhanced Raman scattering imaging, we mapped at the sub-wavelength scale the spatial distribution of the dominant perpendicular component of surface plasmon fields in a metal nanoparticle-film system through spectrally selective and polarization-resolved excitation of the vertical gap mode. The lateral field-extension at the junction, which is determined by the gap-mode volume, is small enough to distinguish a spot size ~0.355λ0 generated by a focused radially polarized beam with high reproducibility. The same excitation and imaging schemes are also used to trace near-field nano-focusing and interferences of surface plasmon polaritons created by a variety of plasmon lenses.

Near-field interference of slit doublet

We study the physical mechanism of near-field interference of slit doublet and explore the distinctive phenomena in near-field interference of slit doublet. The average electric field and average energy-flow density are chosen to describe the near-field distribution rules of the electromagnetic field. The numerical calculations for near-field interference of slit doublet under different illumination conditions are performed according to the finite-difference time-domain method, and the distinctive characteristics of the near-field interference of slit doublet are provided. They include the polarization dependence, the distortion of fringes, and the bifurcation of the fringes. These characteristics are completely different from those of the far-field interference. With the aid of the interference of two slits and of slit and groove, the physical mechanism of polarization dependence of near-field interference is investigated. The fringe distortion of the electric field and the fringe bifurcation of the energy-flow density reflect the amplitude and phase variations of the electromagnetic field. The influences of the slit parameters and the base material on the near-field interference of slit doublet are also discussed. These results may provide us with new insights into the underlying physics of interaction between complex nanostructures and electromagnetic waves.

Femtosecond laser-induced iridescent effect on AZ31B magnesium alloy surface

Both micro-ripples and nano-ripples were firstly reported at AZ31B magnesium alloy surface irradiated by femtosecond laser in atmospheric environment. Iridescent effect was also demonstrated over a large area of the irradiated surface induced by scanning laser beam. Results revealed that the colour effect was mainly attributed to the nano-ripples with broad distribution of periods acting as diffraction gratings, and intensity of the structural colour was greatly influenced by morphology evolution of the micro-ripples with laser processing. It was suggested that near-field interference between surface plasmons polaritons and incident laser light determined the formation of the nano-ripples, and initial surface roughness combing with such interference lead to the formation of the micro-ripples. Potential applications of such effect on Mg alloys and how to apply the technique to other materials with different properties was further proposed.

Femtosecond laser-induced ripple structures on magnesium

Two types of periodic surface structures including micro-ripples and nano-ripples were observed on magnesium after femtosecond laser irradiation, and the color effect was reported by scanning a single laser beam over a large area at near damage threshold fluence. Optical reflectance measurement revealed that the color effect was mainly attributed to the nano-ripples as diffraction gratings, and intensity of the color was affected by morphology of the micro-ripples. AFM and SEM images showed that orientation of the micro-ripples was parallel to laser polarization and the period of the micro-ripples increased with scanning numbers, while the orientation of the nano-ripples was perpendicular to laser polarization and the period of the nano-ripples kept constant as laser sub-wavelength with increasing scanning numbers. These results suggested that formation of the micro-ripples was due to the combined effect of initial surface roughness and near-field interference, while formation of the nano-ripples was caused by the near-field interference between incident light and surface plasmons being excited in the air and metal interface. The potential application of such an effect is further proposed.

Theory of thermal nonequilibrium entropy in near-field thermal radiation

We propose a method to evaluate the entropy density and entropy flux in a vacuum gap between two half-spaces that takes into account influence of near-field effects, i.e., interference, diffraction, and tunneling of waves. The method developed is used to determine the maximum work that can be extracted through near-field radiative transfer between two half-spaces at different temperatures.

Near-Field Interference for the Unidirectional Excitation of Electromagnetic Guided Modes

Wave interference is a fundamental manifestation of the superposition principle with numerous applications. Although in conventional optics, interference occurs between waves undergoing different phase advances during propagation, we show that the vectorial structure of the near field of an emitter is essential for controlling its radiation as it interferes with itself on interaction with a mediating object. We demonstrate that the near-field interference of a circularly polarized dipole results in the unidirectional excitation of guided electromagnetic modes in the near field, with no preferred far-field radiation direction. By mimicking the dipole with a single illuminated slit in a gold film, we measured unidirectional surface-plasmon excitation in a spatially symmetric structure. The surface wave direction is switchable with the polarization.

Singular diffraction-free surface plasmon beams generated by overlapping phase-shifted sources

We propose and experimentally demonstrate the singular surface plasmon beam that presents a dark channel generated by a point dislocation and a long diffraction-free propagation distance up to 70λ(sp). The singular surface beam is the result of the interference of two surface plasmon polariton (SPP) plane waves, which are launched by two coupling gratings with lateral displacement. An aperture-type near-field scanning optical microscope is used to map the intensity distribution of the singular SPP waves. The propagating point dislocation embedded in the beam is shown by full-wave calculations and is later verified by near-field interference in the experiment.

Efficient colour splitters for high-pixel-density image sensors

Colour filters that split light by employing near-field interference effects instead of absorption provide enhanced signal levels for dense, small-pixel image sensors.